This invention relates to a method and apparatus for programmably biasing radio frequency amplifiers.
In conventional wireless communications systems, a single radio frequency (RF) power amplifier can be used to amplify carrier signals modulated with two or more data modulation techniques. For example, in Global System for Mobile (GSM) communications systems, data can be modulated onto carrier signals using either gaussian minimum shift keying (GMSK) or 8-phase shift keying (8-PSK) modulation techniques. Ideally, the bias point of an RF amplifier is optimized to maximize the amplifier""s gain. However, when the same power amplifier is used to amplify carrier signals modulated with two or more data modulation techniques, the amplifier""s optimal bias point depends in part upon the type of signal being amplified. For example, the optimum bias point for an amplifier configured to amplify an 8-PSK modulated carrier signal is about xc2xd the optimum bias point for an amplifier configured to amplify a GMSK modulated carrier signal. Therefore, to optimally bias an RF amplifier configured to amplify carrier signals modulated with two or more data modulation techniques, the bias point needs to be dynamically set depending on how the signal being amplified is modulated.
In one aspect, the invention provides a digitally controlled voltage source configured to programmably control the bias point of a multi-modulation radio frequency amplifier. The bias point of the radio frequency amplifier is set or adjusted when no radio frequency signal is present at the amplifier gate. The digitally controlled voltage source can be configured as a digitally controlled potentiometer with a buffered wiper output. The buffered wiper output can set the bias point of the amplifier.
Aspects of the invention include can include one or more of the following features. The digitally controlled voltage source can be configured to control the bias point of a multi-modulation RF amplifier in an open loop configuration. The digitally controlled voltage source can be implemented as a digitally controlled potentiometer having a plurality of programmable data registers connected to a wiper counter register. The wiper counter register can control the output potential of a wiper terminal. The plurality of data registers can be pre-programmed to optimally bias the RF amplifier when amplifying signals modulated according to a respective one of a plurality of data modulation techniques. One of the plurality of data registers can be pre-programmed to reverse bias the RF amplifier or to turn the RF amplifier off when no radio frequency signal is present. The digitally controlled potentiometer can be configured to receive control signals to write data to the potentiometer""s data registers and to transfer data from any one of the potentiometer""s data registers to the wiper counter register. The control signals can be sent to the potentiometer when no RF signal is present at the gate of the amplifier. The wiper counter register can set the wiper terminal to a position where the wiper terminal has an output potential corresponding to one of the preprogrammed optimal bias potentials.
In another aspect, the invention provides a digitally controlled voltage source configured to control the bias point of a multi-modulation radio frequency amplifier in a closed loop configuration having a remote processor control element. Aspects of the invention can include one or more of the following features. The digitally controlled voltage source can be implemented as a digitally controlled potentiometer having a plurality of programmable data registers connected to a wiper counter register. The wiper counter register can control the output potential of a wiper terminal. The DC bias current of the radio frequency amplifier can be converted to a voltage by a current conversion circuit. The current conversion circuit can be implemented as a resistor coupled to an instrumentation amplifier, where the instrumentation amplifier is configured as a current to voltage converter. The sense voltage can be digitized by an analog-to-digital converter and read into a general purpose microprocessor configured to determine the optimal bias point for the amplifier based on the digitized sense voltage. The microprocessor can determine the optimal bias point, and can program the wiper counter register of the digitally controlled potentiometer so that the wiper terminal has an output potential equal to the determined optimum bias point. The microprocessor can program the wiper counter register when no RF signal is present in the amplifier.
In another aspect, the invention provides a digitally controlled voltage source configured to control the bias point of a multi-modulation radio frequency amplifier in a closed loop configuration under local control. Aspects of the invention can include one or more of the following features. The digitally controlled voltage source can be implemented as a digitally controlled potentiometer. The DC bias current of the radio frequency amplifier can be converted to a voltage by a current conversion circuit such as a resistor coupled to an instrumentation amplifier configured as a current to voltage converter. The sensed voltage can be compared to a reference voltage in a comparator configured to output a logic level establishing a direction to drive the potentiometer""s wiper terminal. The wiper terminal can be incrementally stepped in the direction established by the comparator logic level on the rising or falling edges of a digital clock. The digital clock can be enabled to move the wiper terminal only when no radio frequency signal is present at the gate of the radio frequency amplifier. The wiper terminal output potential is determined by the wiper terminal position, and is continually adjusted when no radio frequency signal is present to maintain an optimal amplifier bias.
Advantages of the invention include the following. The invention allows for the optimum bias point of a radio frequency amplifier to be programmably controlled. The bias point can be set when no radio frequency signal is present at the amplifier gate. The bias point is can be pre-determined and stored in non-volatile memory. The bias point can be programmably controlled to optimize an amplifier configured to amplify radio frequency signals modulated with a plurality of modulation techniques. The bias point can be programmably controlled to compensate an amplifier for the effects of aging, temperature, humidity, and other environmental factors.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.